[PATCH 5.5 083/151] workqueue: dont use wq_select_unbound_cpu() for bound works

[Greg Kroah-Hartman]

From: Hillf Danton <hdanton@xxxxxxxx>

commit aa202f1f56960c60e7befaa0f49c72b8fa11b0a8 upstream.

wq_select_unbound_cpu() is designed for unbound workqueues only, but
it's wrongly called when using a bound workqueue too.

Fixing this ensures work queued to a bound workqueue with
cpu=WORK_CPU_UNBOUND always runs on the local CPU.

Before, that would happen only if wq_unbound_cpumask happened to include
it (likely almost always the case), or was empty, or we got lucky with
forced round-robin placement.  So restricting
/sys/devices/virtual/workqueue/cpumask to a small subset of a machine's
CPUs would cause some bound work items to run unexpectedly there.

Fixes: ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound_cpumask CPUs")
Cc: stable@xxxxxxxxxxxxxxx # v4.5+
Signed-off-by: Hillf Danton <hdanton@xxxxxxxx>
[dj: massage changelog]
Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/workqueue.c |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1411,14 +1411,16 @@ static void __queue_work(int cpu, struct
 		return;
 	rcu_read_lock();
 retry:
-	if (req_cpu == WORK_CPU_UNBOUND)
-		cpu = wq_select_unbound_cpu(raw_smp_processor_id());
-
 	/* pwq which will be used unless @work is executing elsewhere */
-	if (!(wq->flags & WQ_UNBOUND))
-		pwq = per_cpu_ptr(wq->cpu_pwqs, cpu);
-	else
+	if (wq->flags & WQ_UNBOUND) {
+		if (req_cpu == WORK_CPU_UNBOUND)
+			cpu = wq_select_unbound_cpu(raw_smp_processor_id());
 		pwq = unbound_pwq_by_node(wq, cpu_to_node(cpu));
+	} else {
+		if (req_cpu == WORK_CPU_UNBOUND)
+			cpu = raw_smp_processor_id();
+		pwq = per_cpu_ptr(wq->cpu_pwqs, cpu);
+	}
 
 	/*
 	 * If @work was previously on a different pool, it might still be